Magnetrons are widely used as powerful and compact sources for the generation of high power microwaves in a variety of applications. Such applications may include, but are not limited to, microwave ovens, telecommunications equipment, lighting applications, radar applications, and military and weapons applications.
A conventional relativistic magnetron structure is a coaxial vacuum diode with a cathode having a solid cylindrical surface and an anode consisting of cavities forming an azimuthally periodical resonant system. In many designs, resonator cavities of various shapes are cut into the internal surface of the anode, for example, in a gear tooth pattern. During operation, a steady axial magnetic field fills the vacuum annular region between the cathode and anode, and a high voltage pulse is applied between them to provide conditions for microwave generation. Transverse electric-type (TE) eigenmodes of the resonant system are used as operating waves. Usually two types of oscillations are used, the π-mode (with opposite directions of electric field in neighbor cavities) and the 2π-mode (with identical directions of electric field in all cavities). The frequency of the generated microwaves is based in part on the number and shape of the resonator cavities, and the design features of the anode and cathode.
A cross-sectional view of a conventional magnetron is illustrated in FIG. 1. As shown, the magnetron comprises an anode 10, a cathode 20, which is a solid cylindrical structure, and resonator cavities 15. In this example, a waveguide 40 is located in one of resonator cavities 15 in order to extract the generated microwaves. A dielectric 40a also may be present in the waveguide 40. There are other ways known to those skilled in the art for extracting the microwaves as well, such as, for example, axially using diffraction output.
Electrons emitted from the solid cathode 20 form a solid flow drifting around a cathode with a velocity determined by the applied voltage and magnetic field. When the azimuthal phase velocity of one of eigenmodes of the resonant system is close to the azimuthal drift velocity of the electrons, energy of the electrons is transferred to this electromagnetic wave. As the wave gains energy, fields of the wave back-react on the electron charge cloud to produce spatial bunching of the electrons, which in turn reinforces the growth of the wave.
The lifetime of high power relativistic magnetrons is limited by the intense electron bombardment of the cathodes that leads to their destruction. In relativistic magnetrons with explosive electron emission cathodes, the expanding cathode plasma is one of the reasons for decreasing efficiency and pulse shortening (the presence of EM fields increases the expansion of the cathode plasma by an order or magnitude due to plasma heating). Expulsion of adsorbed gases on the cathode (and anode) from one shot to the next also limits the pulse repetition rate between shots.